Field of the Invention
The present invention is related to sensor devices in general and to wearable plethysmograph devices in particular.
Description of the Related Art
In general, a plethysmogram is a volumetric measurement of an organ. An optically determined plethysmogram, referred to as a photoplethysmogram (PPG), may be generated using a pulse oximeter that illuminates the skin and measures changes in light absorption. A conventional pulse oximeter monitors perfusion, i.e., the process of a body delivering blood to a capillary bed in biologic tissue (e.g., dermis and subcutaneous tissue of the skin). With each cardiac cycle, the heart pumps blood to the periphery of a body. Although this pressure pulse is damped by the time it reaches the skin, the pressure is sufficient to distend the arteries and arterioles in the subcutaneous tissue. A light-emitting diode (LED) may illuminate the skin with light and the amount of light either transmitted or reflected may be sensed by a photodiode to detect a change in volume caused by the pressure pulse. Each cardiac cycle may appear as a peak in the sensed signal. Motion may result in artifacts in the signal due to changes in the amount of ambient light leakage, sensor position relative to the skin, blood volume in a localized area, or physiological change due to muscle contraction. Motion-induced artifacts may introduce noise having amplitudes that are substantially greater than the amplitude of the heartbeat signal and may degrade the resulting heart rate measurement. Accordingly, improved techniques for generating a plethysmogram are desired.